In at least one known CT system configuration, an x-ray source projects a fan-shaped beam which is collimated to lie within an X-Y plane of a Cartesian coordinate system and generally referred to as the "imaging plane". The x-ray beam passes through the object being imaged, such as a patient. The beam, after being attenuated by the object, impinges upon an array of radiation detectors. The intensity of the attenuated beam radiation received at the detector array is dependent upon the attenuation of the x-ray beam by the object. Each detector element of the array produces a separate electrical signal that is a measurement of the beam attenuation at the detector location. The attenuation measurements from all the detectors are acquired separately to produce a transmission profile.
In known third generation CT systems, the x-ray source and the detector array are rotated with a gantry within the imaging plane and around the object to be imaged so that the angle at which the x-ray beam intersects the object constantly changes. A group of x-ray attenuation measurements, i.e., projection data, from the detector array at one gantry angle is referred to as a "view". A "scan" of the object comprises a set of views made at different gantry angles during one revolution of the x-ray source and detector. In an axial scan, the projection data is processed to construct an image that corresponds to a two dimensional slice taken through the object.
One method for reconstructing an image from a set of projection data is referred to in the art as the filtered backprojection technique. This process converts the attenuation measurements from a scan into integers called "CT numbers" or "Hounsfield units", which are used to control the brightness of a corresponding pixel on a cathode ray tube display.
To reduce the total scan time required for multiple slices, a "helical" scan may be performed. To perform a "helical" scan, the patient is moved while the data for the prescribed number of slices is acquired. Such a system generates a single helix from a one fan beam helical scan. The helix mapped out by the fan beam yields projection data from which images in each prescribed slice may be reconstructed. In addition to reduced scanning time, helical scanning provides other advantages such as improved image quality and better control of contrast.
In helical scanning, and as explained above, only one view of data is collected at each slice location. To reconstruct an image of a slice, the other view data for the slice is generated based on the data collected for other views. Helical reconstruction algorithms are known, and described, for example, in C. Crawford and K. King, "Computed Tomography Scanning with Simultaneous Patient Translation," Med. Phys. 17(6), November/December 1990.
When performing computed tomography imaging, contrast agents typically are used to enhance image contrast, i.e., to "highlight" an organ of interest from surrounding tissue. Particularly, and with respect to a region of interest, a contrast agent is administered to a patient and the contrast agent is more greatly absorbed by the region of interest than by the other tissues. To obtain images maximizing the contrast between the region of interest and the surrounding tissue, it is preferable to perform a scan during peak contrast agent uptake.
Known methods for attempting to obtain scan data during peak contrast agent uptake in a patient typically require continuously performing low intensity scans, or prep scans, of the region of interest until the scan operator determines that the contrast agent uptake is adequate. After determining that the contrast agent uptake is adequate, the operator initiates the image scan, i.e., a full helical scan.
The known contrast uptake determination methods depend upon the experience of the operator and, particularly with less experienced operators, may result in sub-optimal scans. For example, because of an inherent lag in the CT reconstruction process, images reconstructed using data obtained in a prep scan do not represent the actual state of the contrast uptake when the last view was acquired. Rather, such images represent an averaged contrast uptake during the data acquisition period. Accordingly, at a particular time, the contrast agent uptake may be higher than represented by the prep scan image.
In addition, a considerable delay occurs between initiating the image scan and actually performing the image scan. Specifically, and before performing the scan, the patient must be positioned so that the entire organ volume can be covered, the x-ray tube current level must be increased to the appropriate scanning intensity, and the patient must perform a breath hold. Positioning the patient and increasing the x-ray tube current level typically take several seconds. Similarly, patients typically require several additional seconds to begin a long breath hold.
These delays can result in sub-optimal image scans. Since the contrast agent uptake peaks and diminishes within a significantly short period of time (e.g., 10-30 seconds), the image scan may be performed at a time other than at the time of peak contrast agent uptake.
It would be desirable to perform an image scan during peak contrast agent uptake despite the delays inherent in the scanning process. It also would be desirable to perform such a scan without significantly increasing the costs of known CT systems.